The present invention relates to an optical detector having a heterojunction, and more particularly to an avalanche photodiode.
The development of optical fiber communications technology has achieved rapid progress in recent years, and the transmission wavelength band it uses has been shifting from the 0.8 micron band to the 1 micron band where the transmission loss is minimized. Among optical receivers useful in this wavelength band is a type consisting of an InP substrate and, formed thereon, a crystalline heterojunction comprising InGaAs or InGaAsP and InP. In a photodiode or an avalanche photodiode for high-speed operation having a heterojunction made of such materials, the pn junction, subject to the highest electric field, is formed in InP whose bandgap is wider, in order to avoid a tunnel breakdown apt to occur in InGaAs or InGaAsP whose bandgap is narrow. However, a problem with optical detectors of such a structure is photocurrent components whose response is extremely slow. Thus, in optical detectors of this kind, frequency characteristics are observed to deteriorate in the region from several hundreds of kHz to several tens of MHz, and this phenomenon invites, in wide band optical transmission systems operating in the region of or above several MHz such as those used for optical communications, a sensitivity deterioration in a high frequency band and interference between codes owing to the slow current component.
This problem results from the fact that heterojunction type avalanche photodiodes of prior art are designed solely from the viewpoint of expanding the depletion layer of about 3 microns and above into the light absorbing region, and for the light absorbing layer of InGaAs, for example, a uniform impurity concentration of about 7.times.10.sup.15 cm.sup.-3 is used. As a consequence, the strength of the electric field on the hetero-interface becomes insufficient, inviting the trapping of the carrier as illustrated in FIG. 1(a).
Thus, to describe the band structure of the heterojunction type photodiode illustrated in FIG. 1(a), when a light beam having photon energy of Eg1&gt;h.nu.&gt;Eg2 comes in, light absorption occurs in a layer of a narrow band gap, resulting in the creation of pairs of electron-holes constituting photocarriers. In this case, the electrons move at high speed toward the N side because of the absence of an energy barrier. Meanwhile, the positive holes, faced with the energy barrier of .DELTA.Eg.apprxeq.Eg1-Eg2 in the heterojunction, will be trapped by the hetero-barrier if, as illustrated, the electric field strength is too weak on the hetero-interface to give the holes sufficient kinetic energy to go beyond .DELTA.Eg. The holes thereby trapped will either be absorbed by recombination in the hetero-interface region, or go beyond the barriers by a thermal excitation process to constitute a slow response component having a large time constant.